1. Field of the Invention
Embodiments of the present invention relate generally to orthopedic surgery and, more particularly, to bone screws for management of bone fractures.
2. Description of the Related Art
Surgical techniques for the treatment of bone fractures commonly known and used in the art include external fixation, pinning, and joint replacement. In some situations, each of these techniques can be inadequate for facilitating satisfactory recover of the bone fracture.
A proximal humerus fracture, i.e., a fracture of the humerus near the humeral head, is one such case. Replacement of the shoulder joint with a prosthesis is a complex and invasive procedure that can lead to the death of elderly patients, for whom proximal humerus fractures are common. External fixation of a proximal humerus fracture with one or more humeral plates and bone screws may successfully maintain the correct position of the humerus fragments, but the extensive dissection of soft tissue that is an integral part of this approach leads to high morbidity.
As illustrated in FIG. 1A, rigid, threaded-tip pins 101 may be used for percutaneous fixation of a humeral head 102 of a humerus 100 to bone shaft 103 to treat a proximal humerus fracture 120. Threaded-tip pins 101 are inserted through cortex 105 and into subchondral bone 106 of humeral head 102. In elderly patients who have suffered a proximal humerus fracture, bone of humeral head 102 is generally porous and soft and tends to collapse subsequent to fracture reduction and pinning. The collapse tends to occur adjacent to fracture 120, where the bone is also most fragmented. Further, the soft bone of humeral head 102 does not hold the tips of threaded-tip pins 101 securely. Because threaded-tip pins 101 are held in place at a single point that is relatively far from subchondral bone 106, i.e., penetration point 104 of cortex 105, threaded-tip pins 101 are free to angulate or tilt about penetration point 104 and therefore offer little stability to humeral head 102. Collapse or fragmentation of the bone of humeral head 102 and poor fixation of threaded-tip pins 101 in subchondral bone 106 allow further movement between humeral head 102 and bone shaft 103. A displacement force is applied by muscles to bone shaft 103, further causing movement at fracture 120. Tilting of threaded-tip pins 101 allows humeral head 102 to be angulated and displaced from bone shaft 103, as illustrated in FIG. 1B. Such angulation and displacement require additional surgery for satisfactory recovery of proximal humerus fracture 120.
FIG. 2A illustrates another prior art method for percutaneous fixation of the humeral head 202 of a humerus 200 to a bone shaft 203 to treat a proximal humerus fracture 230. First, humeral head 202 is returned to its proper position on bone shaft 203, using methods standard to the art of orthopedic surgery. Then, one or more fully-threaded K-wires 201 are introduced into the intramedullary cavity 210 of humerus 200 through an opening 204 in the antero-lateral cortex 205 of humerus 200. For clarity, only one fully-threaded K-wire 201 is depicted in FIG. 2A. Fully-threaded K-wires 201 are then advanced into the intramedullary cavity 210, along far cortex 207, and threaded into the subchondral bone 206. Because each fully threaded K-wire 201 is supported by far cortex 207 and is not free to angulate or tilt, humeral head 202 is not subject to angulation or displacement if subchondral bone 206 collapses after fixation or if fixation of K-wire 201 in humeral head 202 is suboptimal. However, collapse of humeral head 202 does produce other complications when the method illustrated in FIG. 2A is used to fixate proximal humeral fracture 230.
FIG. 2B illustrates humerus 200 after fixation with one or more fully-threaded K-wires 201. As shown, collapse of bone adjacent to fracture 230 results in penetration of the shoulder joint by fully threaded k-wire 201. This is because the threads on the shaft of fully threaded K-wire 201 engage the edges of opening 204 and hold fully threaded K-wire 201 in place as bone adjacent to fracture 230 collapses. Even if opening 204 is over-sized relative to the outer diameter of fully threaded K-wire 201, the threads on the shaft of fully threaded K-wire 201 engage the edge of opening 204 due to loading caused by the elastic bend in fully threaded K-wire 201. In addition, fully threaded K-wire 201 has limited holding power in the relatively soft material of subchondral bone 206, since fully threaded K-wire 201 must have a relatively small diameter in order to have the necessary flexibility for insertion into humerus 200. The limited holding power of fully threaded K-wire 201 further encourages penetration of the joint as bone adjacent to fracture 230 collapses. Joint penetration by fully threaded K-wire 201 can lead to unwanted cartilage and bone damage and requires immobilization of the joint for the duration of treatment, i.e., until proximal humeral fracture 230 has healed and fully threaded K-wire 201 has been removed.
An additional complication associated with the approach illustrated in FIG. 2A is K-wire breakage. Some small diameter models of fully threaded K-wires known in the art have sufficient flexibility for use as fully threaded K-wire 201 as described above. However, it is known that the bending moment exerted on fully threaded K-wire 201 when rotationally inserted into humerus 200 can result in breakage of fully threaded K-wire 201 in the intramedullary cavity 210, which is highly undesirable. This breakage is related to the notch effect of the threads on the shaft of K-wire 201. Rotation of the elastically bent K-wire 201 during insertion causes cyclic loading that accentuates the notch effect.
A further complication of K-wire usage is the penetration of far cortex 207. If the tips are too sharp, i.e., the included angle of the point is too small, K-wire 201 tends to penetrate far cortex 207 rather than slide or rotationally advance along the inner surface of far cortex 207 as it is rotationally inserted. This problem is especially notable for trocar point K-wires. A further reason for penetration of far cortex 207 is that there is lacking instrumentation and methodology which can direct the K-wires away from entry into far cortex 207.
Accordingly, there is a need in the art for devices and methods for the management of bone fractures which prevent angulation and displacement of bone fragments, do not result in joint penetration by repair devices due to collapse of bone of the head, avoid breakage of repair devices inside the fractured bone, and avoid penetration of the far cortex by repair devices during their insertion.